1. Field of the Invention
The present invention relates to a color cathode-ray tube, and more particularly, to a color cathode-ray tube in which mislanding of beam attributable to thermal expansion of a shadow mask and impact is reduced by changing the configuration or curvature of the effective surface of the mask and/or the inner surface of an effective region of a panel.
2. Description Of The Related Art
In general, a color cathode-ray tube comprises a phosphor screen formed of three color phosphor layers and a shadow mask facing the screen. Three electron beams emitted from an electron gun are sorted by means of the shadow mask, and a color image is displayed on the phosphor screen.
The color cathode-ray tube comprises a rectangular panel having an effective area whose inner surface is formed essentially of a curved surface, and the phosphor screen is formed on the inner surface of the effective area. On the other hand, the shadow mask includes a mask body, which has a substantially rectangular effective surface, and a mask frame attached to the peripheral portion of the mask body. The effective area of the mask body is in the form of a curved surface corresponding to the inner surface of the panel in configuration and a number of electron beam apertures, through which the electron beams pass, are formed in this curved surface. The shadow mask is supported on the inside of the panel in a manner such that elastic support members, to which the mask frame is attached, are fitted and retained on stud pins on the panel.
In order to display a color image having good color purity on the phosphor screen, in the color cathode-ray tube constructed in this manner, the three electron beams passed through their corresponding apertures of the shadow mask must be landed exactly on the three color phosphor layers which constitute the phosphor screen. To attain this, the distance between the panel and the shadow mask, especially the distance (value q) between the inner surface of the effective area of the panel and the effective surface of the mask, should be kept within an allowable range.
Conventionally, however, the body of the shadow mask is formed of a thin carbon steel sheet, and the quantity of those electron beams which reach the phosphor screen after passing through the apertures in the effective surface of the mask body is not greater than one third of the quantity of the electron beams emitted from the electron gun, that is, most of the electron beams impinge on the mask. As a result, the shadow mask is heated to undergo thermal expansion, and the thin curved mask body, in particular, is subjected to doming such that it bulges toward the phosphor screen. If the height of the bulge attributable to the doming exceeds the allowable range of the value q, the electron beams land on the three color phosphor layers with a lag, thereby deteriorating color purity. This mislanding attributable to the thermal expansion of the shadow mask varies depending on the electric current of the electron beams, the size and duration of an image pattern, etc.
One such mislanding, attributable to the thermal expansion of the shadow mask, lasts for a relatively long period of time (30 minutes or more) before the temperature of the mask body, which, thinner than the mask frame, is heated in the initial stage of the operation of the color cathode-ray tube, is transmitted to the mask frame to establish a thermal equilibrium such that the respective temperatures of the mask frame and the mask body are substantially equal. This mislanding can be effectively corrected by interposing a bimetal element between the mask frame and elastic support members for supporting the shadow mask, as is described in Jpn. Pat. Appln. KOKOKU Publication No. 44-3547, for example. If a high-luminance image is locally displayed for a relatively short period of time, however, the shadow mask is subjected to a local thermal bulge, and the resulting local mislanding cannot be corrected by means of the bimetal element.
A rectangular pattern for the mislanding attributable to the thermal expansion of the shadow mask was drawn on the phosphor screen by means of a signal generator, and the size of the landing lag was measured varying the shape and originating position of the pattern. Thereupon, it was confirmed that the landing lag was small when a high-current, high-luminance rectangular pattern was formed substantially over the whole region of the phosphor screen. It was also ascertained that the greatest landing lag was caused when a high-current, high-luminance elongated rectangular pattern was formed converging a little from the left- or right-hand end (with respect to the horizontal axis or x-axis) of the phosphor screen.
These phenomena can be easily understood from the description to follow.
First, a TV set is generally designed so that an average anode current applied to the cathode-ray tube, that is, a current flowing through the anode for the whole picture, should not exceed a given value. When a large high-luminance rectangular pattern is formed on the phosphor screen in the aforesaid manner, therefore, the beam current for each unit area of the shadow mask is lower, and the temperature rise of the mask is smaller, than when the a small high-luminance rectangular pattern is formed.
Secondly, when a high-luminance pattern is formed on the central portion of the phosphor screen, a landing lag cannot be easily caused even though the shadow mask is subjected to thermal expansion. As the originating position of the pattern shifts from the center of the phosphor screen toward the left- and right-hand ends thereof, the thermal expansion of the shadow mask appears more frequently as a landing lag. At the opposite ends the screen, however, the body of the shadow mask is fixed by means of the mask frame, so that only a small deformation is caused by the thermal expansion. Thus, if a high-luminance pattern is formed converging a little from the left- or right-hand end of the phosphor screen, that is, if a region intermediate between the center and the horizontal end of the shadow mask, especially a region just outside the point halfway between the center and the horizontal end of the mask, is heated, then the mask undergoes a substantial thermal expansion, resulting in the greatest landing lag.
When the shadow mask is located in a normal position, an electron beam passing through one aperture which is situated a little nearer to the center of the mask than the horizontal end thereof lands exactly on its corresponding phosphor layer. If a high-luminance image is displayed by means of a high-current electron beam passing near the specific aperture, however, the shadow mask is subjected to thermal expansion in the vicinity of the aperture by impingement with the electron beam. This thermal expansion shifts the position of the electron beam aperture, so that the electron beam passing through this shifted aperture ceases to land on the specified phosphor layer.
In most of modern color cathode-ray tubes, in particular, the effective area of the panel is flat, so that the effective surface of the body of the shadow mask is also flat. Accordingly, the shadow mask is more easily deformed by thermal expansion due to the impingement with the electron beam, and a substantial landing lag is liable to be caused.
Disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 61-163539 and 61-88427 is means for restraining the mislanding by changing the configuration of a flat shadow mask. According to color cathode-ray tubes which combines a flat panel and a flat shadow mask, however, a satisfactory effect cannot be obtained from the configuration of the shadow mask described in these publications. Thus, in the modern color cathode-ray tubes, the panel and the shadow mask are flatter than the ones described in the above publications, and the landing lag attributable to thermal expansion of the mask by impingement electron beams is greater. In consequence, the landing lag cannot be fully corrected with use of the shadow mask configuration described in the aforesaid publications.
Disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 64-17360 and 1-154443 is means for restraining the landing lag attributable to thermal expansion of the shadow mask by changing the curvature of the panel. If the curvature of the panel is changed, as described in these publications, however, a satisfactory effect cannot be obtained for a substantially spherical flat panel which ensures a natural agreeable reflection on its outer surface, and has recently started to be used practically.
The color cathode-ray tube whose panel and shadow mask have flat effective surfaces involves the following problems, as well as the thermal expansion of the shadow mask.
In the color cathode-ray tube whose panel has a flat effective surface, the body of the shadow mask may be formed of a low-expansion material, such as Invar, besides a low-carbon steel sheet which is used for the shadow mask of a conventional color cathode-ray tube. Normally, the shadow mask body is press-molded to have a predetermined curved surface after apertures are formed therein by photo-etching. In this case, a large-curvature mask body can be subjected to appropriate plastic deformation to obtain a necessary mechanical strength as it is press-molded. However, a flat mask body cannot be subjected to satisfactory plastic deformation, and inevitably involves local low-strength portions. In other words, flattening the effective surface of the shadow mask results in reduction in the deformation and elongation of the mask during the press molding operation. Therefore, some portions of the shadow mask cannot be molded to a region for plastic deformation, remaining in a region for elastic deformation. In the case of a shadow mask which has a substantially rectangular effective surface, in particular, its short sides, which are situated at a distance in the direction of the horizontal axis from the center, are more distant from the center than its long sides, which are situated at a distance in the direction of the vertical axis from the center. Accordingly, those horizontal end portions of the mask which are situated a little nearer to the center than the short sides are the most fragile portions, which are deformed by impact or the like. Thus, the portions situated a little nearer to the center than the short sides, on the horizontal axis of the effective surface of the shadow mask, are distant from the center of the mask, and, unlike diagonal portions, are not surrounded by the skirt portion of the mask. Therefore, those horizontal end portions cannot undergo perfect plastic deformation during the press molding operation, and remain in the elastic deformation region. Consequently, they fail to be formed into previously designed curved surfaces, and their strength is lowered. Moreover, those portions easily resonate, causing a color drift.